Field effect transistors (FETs) are the basic building block of today's integrated circuit. Such transistors can be formed in conventional bulk substrates (such as silicon) or in semiconductor-on-insulator (SOI) substrates.
State of the art metal oxide semiconductor (MOS) transistors are fabricated by depositing a gate stack material over a gate dielectric and a substrate. Generally, the MOS transistor fabrication process implements lithography and etching processes to define the conductive, e.g., poly-Si, gate structures. The gate structure and substrate are thermally oxidized, and, after this, source/drain extensions are formed by implantation. Sometimes the implant is performed using a spacer to create a specific distance between the gate and the implanted junction. In some instances, such as in the manufacture of an n-FET device, the source/drain extensions for the n-FET device are implanted with a thinner spacer than the p-FET device.
A thicker spacer is typically formed after the source/drain extensions have been implanted. The deep source/drain implants are then performed with the thick spacer present. High temperature anneals are performed to activate the junctions after which the source/drain and top portion of the gate are generally silicided. Silicide formation typically requires that a refractory metal be deposited on a Si-containing substrate followed by a process to produce the silicide material. The silicide process forms low resistivity contacts to the deep source/drain regions and the gate conductor.
High integration density can reduce manufacturing costs. In order to be able to make integrated circuits (ICs), such as memory, logic, and other devices, of higher integration density than currently feasible, one has to find a way to further downscale the dimensions of field effect transistors (FETs), such as metal oxide semiconductor field effect transistors. The downscaling of transistor dimensions allows for improved performance as well as compactness, but such downscaling has some device and yield degrading effects. Generation improvements for high performance MOSFETs are obtained by decreasing the line width (i.e., channel length), reducing the gate oxide thickness, and decreasing the source/drain extension resistance. Smaller transistor line width results in less distance between the source and the drain. This results in faster switching speeds for complementary metal oxide semiconductor (CMOS) circuits.
Unfortunately, reducing the channel length of a transistor also increases short channel effects, as well as “edge effects” that are relatively unimportant in long channel transistors. One example of a short channel effect includes, among other aspects, an increased drain-to-source leakage current when the transistor is supposed to be in the “off” or non-conductive state, due to an enlarged drain-to-body and source-to-body junction depletion region relative to the shorter channel length. In addition, one of the edge effects that may also adversely influence transistor performance is the gate-to-source/drain capacitance. A part of this parasitic fringe capacitance can be effectively increased via transistor gain factor, and is known as Miller capacitance. In CMOS applications, the Miller capacitance is an amplification of a gate-to-drain capacitance.
It is known in the semiconductor industry that a halo implantation can be used to control short channel effects. Although halo implantation can help to control short channel effects, the presence of the same in the inversion layer degrades the mobility of the carriers within the inversion layer and oftentimes reduces the device performance. The aforementioned problems with halo implantation are increased with shorter gate lengths since a higher concentration of halo profile is typically needed.
In view of the above, there is a need for providing a new and improved MOSFET structure that exhibits high performance and improved short channel effects.